In a UMTS (Universal Mobile Telecommunications System) network, attempts are made to optimize features of the system, which are based on W-CDMA (Wideband-Code Division Multiple Access), by adopting HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access), for the purposes of improving spectral efficiency and improving the data rates. With this UMTS network, long-term evolution (LTE) is under study for the purposes of further increasing high-speed data rates, providing low delay, and so on (non-patent literature 1).
In a third-generation system, it is possible to achieve a transmission rate of maximum approximately 2 Mbps on the downlink by using a fixed band of approximately 5 MHz. In an LTE system, it is possible to achieve a transmission rate of about maximum 300 Mbps on the downlink and about 75 Mbps on the uplink by using a variable band which ranges from 1.4 MHz to 20 MHz. With the UMTS network, successor systems of LTE are also under study, for the purpose of achieving further broadbandization and higher speed (for example, LTE-advanced (“LTE-A”)). The system band of an LTE-A system includes at least one component carrier (cell), where the system band of the LTE system is one unit. Gathering a plurality of components carriers (CCs) into a wide band in this way is referred to as “carrier aggregation” (CA).
In the uplink of the LTE-A system, application of SC-FDMA as a radio access scheme is under study. Consequently, a study is in progress to selectively transmit retransmission control information (ACK/NACK, and so on) corresponding to downlink signals (PDSCH signals) that are transmitted from a plurality of downlink CCs, respectively, from a single CC (for example, a P-Cell (Primary-Cell)), so as to maintain the characteristics of uplink single-carrier transmission. In this case, to feed back a plurality of retransmission control signals in response to the downlink signal of each CC, a user terminal needs to control the allocation of PUCCH resources. In Rel. 10 LTE, application of channel selection and so on, as allocation of PUCCH resources to retransmission control signals for two CCs is under study (non-patent literature 2).
In radio communication, as uplink and downlink duplexing methods, there are frequency division duplexing (FDD), which divides the uplink and the downlink by frequency, and time division duplexing (TDD), which divides the uplink and the downlink by time. In Rel. 10 LTE, when executing carrier aggregation in TDD, as shown in FIG. 1A, the ratio between uplink subframes and downlink subframes (Transmission Time Intervals: TTIs) is the same in all component carriers. In Rel. 11 LTE, considering application of a heterogeneous network and so on, as shown in FIG. 1B, changing the ratio between uplink subframes and downlink subframes in each component carrier when executing carrier aggregation in TDD is under study.
In the event carrier aggregation is applied, as shown in FIG. 2, for example, multiplexing and transmitting downlink control information (DCI 2) for a downlink shared channel to be transmitted by a component carrier CC 2 (an S-Cell (Secondary-Cell)), on a downlink control channel (PDCCH) of another component carrier CC 1 (P-Cell (Primary-Cell)), is under study (cross-carrier scheduling). When this takes place, a DCI configuration, in which a carrier indicator (CI) is added in order to identify for which component carrier's (between CC 1 and CC 2) downlink shared channel the downlink control information (DCI 2) provides information, is adopted.